Unfortunately, Windows Media Player on all versions up to Windows 8.1 will not play 4k video files. If you've recorded a 4k file from Reflector 3 and attempt to play the file with Windows Media Player, you may see the following error:
The file is still playable! To play the file, you will need to use VLC from VideoLAN. VLC is a trusted and robust media player that will correctly play your recorded files.